The organic carbon (C org ) stored in seagrass meadows is globally significant and could be relevant in strategies to mitigate increasing CO 2 concentration in the atmosphere. Most of that stored C org is in the soils that underlie the seagrasses. We explored how seagrass and soil characteristics vary among seagrass meadows across the geographic range of turtlegrass ( Thalassia testudinum ) with a goal of illuminating the processes controlling soil organic carbon (C org ) storage spanning 23° of latitude. Seagrass abundance (percent cover, biomass, and canopy height) varied by over an order of magnitude across sites, and we found high variability in soil characteristics, with C org ranging from 0.08 to 12.59% dry weight. Seagrass abundance was a good predictor of the C org stocks in surficial soils, and the relative importance of seagrass-derived soil C org increased as abundance increased. These relationships suggest that first-order estimates of surficial soil C org stocks can be made by measuring seagrass abundance and applying a linear transfer function. The relative availability of the nutrients N and P to support plant growth was also correlated with soil C org stocks. Stocks were lower at N-limited sites than at P-limited ones, but the importance of seagrass-derived organic matter to soil C org stocks was not a function of nutrient limitation status. This finding seemed at odds with our observation that labile standard substrates decomposed more slowly at N-limited than at P-limited sites, since even though decomposition rates were 55% lower at N-limited sites, less C org was accumulating in the soils. The dependence of C org stocks and decomposition rates on nutrient availability suggests that eutrophication is likely to exert a strong influence on carbon storage in seagrass meadows.